Cellular wireless is an increasingly popular means of personal communication in the modern world. People are using cellular wireless networks for the exchange of voice and data over cellular telephones, Personal Digital Assistants (“PDAs”), cellular modems, and other devices. In principle, a user can seek information over the Internet or call anyone over a Public Switched Telephone Network (“PSTN”) from any place inside the coverage area of the cellular wireless network.
In a typical cellular wireless system, an area is divided geographically into a number of cell sites provided by a radio access network (RAN). The RAN typically comprises one or more base transceiver stations (BTSs), each of which has one or more antennas that radiate to define a radio frequency (RF) radiation pattern. The BTS(s) of the RAN may then be coupled with a base station controller (BSC) or radio network controller (RNC), which may in turn be coupled with a telecommunications switch or gateway, such as a mobile switching center (MSC) or packet data serving node (PDSN) for instance. The switch or gateway may then provide connectivity with a transport network, such as the public switched telephone network (PSTN) or the Internet for instance.
When a mobile station (such as a cellular telephone, a wirelessly equipped PDA or personal computer, or another suitably equipped device) is positioned in a cell, the mobile station communicates via an RF air interface with the BTS of the cell. Consequently, a communication can be established between the mobile station and another entity, via the air interface and the RAN. Such a communication may be referred to as a “call,” whether the communication is a traditional voice “call” or a more advanced data session, generally regardless of its content.
With the explosive growth in demand for wireless communications, the level of call traffic in most cell sites has increased drastically over recent years. To help manage the call traffic, most cells in a wireless network are usually further divided geographically into a number of sectors (which can be visualized ideally as pie pieces), each defined respectively by radiation patterns from directional antenna components of the respective BTS, or by respective BTS antennae.
In a Code Division Multiple Access (CDMA) wireless network and perhaps in other types of networks, each cell employs one or more carrier frequencies, and each sector is distinguished from adjacent sectors by a pseudo-random number offset (PN offset). Further, each sector may concurrently communicate on multiple different channels, distinguished by “Walsh codes”. When a mobile station operates in a given sector, communications between the mobile station and the BTS of the sector are carried on a given frequency and are encoded by the sector's PN offset and, perhaps, a given Walsh code.
According to well known industry standards, a mobile station can communicate with a number of “active” sectors at a time. Depending on the system, the number of active sectors may be up to three or six, for instance. The mobile station receives largely the same signal from each of the active sectors and, on a frame-by-frame basis, may select the best signal to use.
A mobile station maintains in its memory a list of the sectors in its “active” set. In addition, it maintains in its memory a list of “candidate” sectors (e.g., up to six), which are those sectors that are not yet in the active set but that have sufficient signal strength that the mobile station could demodulate signals from those sectors. Further, the mobile station maintains a list of “neighbor” sectors, which are those sectors not in the active set or candidate set but are in close vicinity to the mobile station. All other possible sectors are members of a “remaining” set.
In existing systems, to facilitate a determination of which sectors should be in the mobile station's active set, all base stations emit a pilot channel signal, typically at a power level higher than other downlink signals. A mobile station then constantly measures the strength (Ec/Io, i.e., energy versus spectral density) of each pilot that it receives and notifies a primary base station (a base station currently serving the mobile station) when pilot strength falls above or below designated thresholds. The base station, in turn, provides the mobile station with an updated list of active pilots.
In one arrangement, for instance, the base station may initially transmit to the mobile station (e.g., over a downlink control channel or traffic channel) a Handoff Direction Message (HDM), containing parameters such as (i) the PN offsets of the sectors in the active set and (ii) the following handoff parameters that relate to pilot signal strength:                T_ADD: Threshold pilot strength for addition to the active set (e.g., −14 dB)        T_COMP: Difference in signal strength from an active set pilot (e.g., 2 dB)        T_DROP: Threshold pilot strength for removal from the active set (e.g., −16 dB)        T_TDROP: Time for which an active set pilot falls below T_DROP to justify removal from the active set (e.g., 2 seconds)Additionally, the base station may initially provide the mobile station with a Neighbor List Update Message (NLUM), which identifies the “neighbor” sectors for the current active set.        
The mobile station may then monitor all of the pilot signals that it receives, and the mobile station may determine if any neighbor pilot exceeds T_ADD by T_. If so, the mobile station may add the pilot to its “candidate” set and send a Pilot Strength Measurement Message (PSMM) to the base station, indicating the estimated Ec/Io for the pilot. Depending on current capacity and other issues, the base station may then send an HDM to the mobile station, listing the pilot as a new member of the active set, or directing the mobile station to add the pilot to the active set. Upon receipt of the HDM, the mobile station would then add the pilot to its active set as instructed, and the mobile station would send a Handoff Completion Message (HCM) to the base station, acknowledging the instruction, and providing a list of the pilots (PN offsets) in its active set.
Similarly, if the mobile station detects that the signal strength of a pilot in its active set drops below T_DROP, the mobile station may start a handoff drop timer. If T_TDROP passes, the mobile station may then send a PSMM to the base station, indicating the Ec/Io and drop timer. The base station may then respond by sending an HDM to the mobile station, without the pilot in the active set, or directing the mobile station to remove the pilot from the active set. The mobile station would then receive the HDM and responsively move the pilot to its neighbor set and send an HCM to the base station.
When a mobile station is actively engaged in a call, if RF signal strength received by the mobile station on each on each sector of its active set becomes too low and the mobile station does not have a chance to add a sufficiently strong sector to its active set, the RF link between the mobile station and RAN will be effectively cut off, which will thereby disconnect or “drop” the call.
This can happen for a number of reasons. For example, one or more sectors may be missing from the neighbor list associated with the mobile station's current active set sectors. In certain systems, a RAN may preclude handoff to a sector not in the mobile station's neighbor list. Thus, if the mobile station that is actively engaged in a call moves into an area where the mobile station loses contact with its active set sectors and seeks to hand off to a sector that is not in its neighbor list, the mobile station may experience a call drop. As another example, if the mobile station is moving quickly or traveling through RF obstructions, the mobile station may detect a strong pilot on a sector in its neighbor list but may not have time to send a PSMM for that pilot and receive an HDM directing handoff to that pilot before the mobile station loses contact with its current active set sectors. In that case as well, the mobile station may experience a call drop. Other situations may give rise to call drops as well.